This invention relates to electrochemical analysis and more particularly to gas detection electrochemical cells.
Gas detection electrochemical cells are known in which a detection electrode is arranged in a body containing an electrolyte. A gas permeable membrane is provided to separate the electrode and electrolyte from the sample and to permit passage of gas to a film of electrolyte, between the membrane and the electrode, which connects the detection electrode to a reference electrode.
A number of different gases have been measured using such systems, and a variety of electrolytes have been employed. Frant et al. U.S. Pat. No. 3,859,191 describes an HCN-sensing electrochemical cell employing an electrolyte containing KAg(CN).sub.2. Riseman et al. U.S. Pat. No. 3,897,315 describes an H.sub.2 S-sensing electrochemical cell employing an ion-specific reference electrode. Krueger et al. U.S. Pat. No. 3,803,006 describes an SO.sub.2 -sensing electrochemical cell employing an electrolyte containing Na.sub.2 SO.sub.3. Strickler et al. U.S. Pat. No. 3,649,505 describes an NH.sub.3 -sensing electrochemical cell employing an electrolyte containing NH.sub.4 Cl. Riseman et al. U.S. Pat. No. 3,830,718 describes an NH.sub.3 -sensing electrochemical cell employing an electrolyte containing ammonium picrate. Synnott et al. U.S. Pat. No. 4,105,525 describes an NH.sub.3 -sensing electrochemical cell employing an electrolyte containing 5,5'-Nitrilodibarbituric acid ammonium salt.
The present invention provides improved electrochemical cells for detecting any of a variety of chemical gases.
In general the invention features, in a first aspect, an electrochemical cell for detecting, in a gaseous or liquid sample, any of the gases SO.sub.2, NO.sub.x (nitrogen oxide), HNC, NH.sub.3, formic acid, acetic acid, or CO.sub.2. The cell includes a body and within the body a detection electrode, a reference electrode, a liquid electrolyte connecting the electrodes, and a membrane permeable to the gas in close proximity to the detection electrode and arranged to separate the electrodes and the electrolyte from the sample. The electrolyte is buffered to a pH value such that the electrochemical cell gives a slope of response to the gas within 25% of a Nernstian response. (As is well known, a Nernstian response is a change in measured potential, with a ten-fold change in concentration, which ideally fits the Nernst equation. For a monovalent ion, the potential change is 59.16 mv at 25.degree. C.; a response within 25% of this is between 44.37 and 73.95 mv). The electrolyte is initially (prior to use) essentially (over 99%, v/v) free of any salt capable of dissociating to form in said electrolyte a weak acid or weak base (other than H.sub.3 O.sup.+ or OH.sup.-) which forms upon dissociation of the gas being measured. The reference electrode in contact with the electrolyte is operative at the pH value of the liquid electrolyte.
In preferred embodiments of the above first aspect of the invention, the liquid electrolyte is an aqueous buffer solution containing, at a concentration of between 0.5% and 70%, v/v, a water-miscible solvent having a dielectric constant of at least 10 and being non-interfering with the detection and reference electrodes.
In other preferred embodiments of the above first aspect of the invention, the detection electrode is a pH electrode or an ion-specific electrode selective for an ionic species formed in the electrolyte upon dissociation of the gas therein; and the reference electrode is a silver-based electrode, a platinum-based electrode, or an ion selective electrode the primary electrode of which is in contact with the electrolyte, which electrolyte contains the ionic species for which the ion-specific reference electrode is selective. The reference electrode is selected so that the ionic species for which it is selective is substantially (over 90%) uncomplexed at the pH of the electrolyte.
In a second aspect the invention features, in general, an electrochemical cell for detecting H.sub.2 S in a gaseous or liquid sample, the cell including a body and within the body a detection electrode, a reference electrode which is not an ion selective electrode, a liquid electrolyte connecting the electrodes, and a membrane permeable to H.sub.2 S in close proximity to the detection electrode and arranged to separate the electrodes and the electrolyte from the sample. The liquid electrolyte is an aqueous buffer solution buffered to a pH value such that the electrochemical cell gives a slope of response to the H.sub.2 S within 25% of a Nernstian response. The reference electrode in contact with the electrolyte is operative at the pH value of the liquid electrolyte.
In preferred embodiments of the above and second aspect of the invention, the liquid electrolyte contains a water-miscible solvent having a dielectric constant of at least 10 and being non-interfering with the detection and reference electrodes; the solvent is present in a concentration of at least 25%, v/v,; the detection electrode is a pH electrode or sulfide ion selective electrode; the reference electrode is a silver-based electrode or a platinum-based electrode; the reference electrode is selected so that the ionic species to which it responds is substantially (over 90%) uncomplexed at the pH of the electrolyte; and the electrolyte is initially (prior to use) essentially (over 99%, v/v) free of any salt capable of dissociating in the electrolyte to form HS.sup.-.
We have discovered relationships in the above systems between the pK of the gas being detected and the optimum pH of the liquid electrolyte. One relationship is similar for all of the protonated gases, and another is similar for all of the non-protonated gases. Consequently, in preferred embodiments of all of the above aspects of the invention, where the gas being detected is one of the protonated gases H.sub.2 S, HCN, formic acid, or acetic acid, the pH of the liquid electrolyte is within 2.3 units of the pK of the gas, most preferably within 1.0 unit of the pK of the gas.
Where the gas being detected is one of the non-protonated gases SO.sub.2, NH.sub.3, NO.sub.x, or CO.sub.2, the pH of the liquid electrolyte is at least 1.7 units above or below the pK of the gas, most preferably at least 2.7 units above or below the pK of the gas.
The electrochemical cells of the invention have the advantages of stability and economy. For example, the SO.sub.2 electrode is, as far as is known, the first commercially practical device, prior devices having contained salts, dissociating to form HSO.sub.3.sup.- or SO.sub.3.sup.--, which were extremely unstable and tended to decompose during storage. Furthermore, where the electrolyte is essentially free of any salt capable of dissociating to form the weak acid or weak base which forms upon dissociation of the gas being measured, an ion selective electrode can be used as the detection electrode, an option not available when the electrolyte already contains the ion to be measured. This advantage can be particularly important in situations in which two gases with close pK values are likely to be present in one sample, so that a pH change observed using a pH electrode as the detection electrode could be attributed to either of the gases, and an ion selective detection electrode must therefore be used to give an accurate result. A further advantage is that the absence of such a salt can in some cases (e.g. in the case of HCN detection) allow the avoidance of toxic or corrosive substances.